1. Field of the Invention
This invention relates to light emitting diodes (LEDs) and more particularly to high light extraction efficiency gallium nitride based LEDs via surface roughening.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Gallium nitride (GaN) based wide band gap semiconductor LEDs have been available for almost 15 years. The progress of LED development has brought about great changes in LED technology, with the realization of full-color LED displays, LED traffic signals, white LEDs, and so on.
High efficiency white LEDs have gained much interest as possible replacements for fluorescent lamps—the luminous efficacy of white LEDs (130-150 lumens/watt [1]) already surpasses that of ordinary fluorescent lamps (75 lumens/watt). Nevertheless, current commercially available wurzite nitride based LEDs are characterized by the presence of polarization-related electric fields inside multi-quantum wells (MQWs), for their [0001] c-polar growth orientation. The discontinuities in both spontaneous and piezoelectric polarization at the heterointerfaces result in internal electric fields in quantum wells which cause carrier separation (quantum confined Stark effect (QCSE)) and reduce the radiative recombination rate within quantum wells [2-5].
To decrease these polarization-related effects, growing III-nitride devices on the nonpolar planes, viz, the (1-100) m-plane or the (11-20) a-plane, has been demonstrated [6-7]. Another approach to reduce, and possibly eliminate those effects, is to grow III-nitride devices on crystal planes that are inclined with respect to the c-direction, i.e., semipolar planes. Devices grown on different semipolar planes, including (10-1-1), (10-1-3), (11-22) and others, have also been demonstrated [8-10]. These planes have reduced polarization discontinuity in heterostructures compared with the c-plane III-nitride materials; and for semipolar planes oriented ˜45 degree from the c-plane, there is no polarization discontinuity in InGaN/GaN heterostructures [5]. Recently, with the advent of high quality freestanding GaN substrates, high performance nonpolar and semipolar LEDs with peak emission wavelengths ranging from 407 nm to 513 nm on nonpolar m-plane, semipolar (10-1-1), and (11-22) freestanding GaN substrates have been reported. The performance highlights of those LEDs are summarized in Table 1 [11-15]. Those devices show greatly reduced polarization-related electric fields in the quantum wells, which enables one to employ thicker quantum wells inside an LED, which is believed to be crucial for devices operating under high currents. Therefore, LEDs grown on nonpolar and semipolar oriented GaN substrates hold great promise for commercially useful solid-state lighting applications and could be commercially viable as high quality freestanding GaN substrates become more available.
TABLE 1Summary of the performance of recently reported semipolar and nonpolar LEDs.ExternalQuantumEfficiency atPeak EmissionOutput Power at20 mA driveWavelengthCrystal Orientation20 mA drive currentcurrent407 nm (violet-blue),Nonpolar m-plane,23.7 mW,38.9%,411 nm (violet-blue)Semipolar (10-1-1)20.58 mW33.9%plane444 nm (blue)Semipolar (10-1-1)16.21 mW (under29% (underplanepulsed operations,pulsed operations,10% duty cycle)10% duty cycle)489 nm (blue-green)Semipolar (11-22)9 mW (under pulsed18% (underplaneoperations, 10% dutypulsed operations,cycle)10% duty cycle)516 nm (green)Semipolar (11-22)5 mW10.5%plane
Current techniques to improve the efficiency of an LED fall under two distinct categories: increasing the internal quantum efficiency or the extraction efficiency.
Increasing the internal quantum efficiency, determined by crystal quality and epitaxial layer structure, could be rather difficult. A typical internal quantum efficiency value for blue LEDs is more than 70% [16] and an ultraviolet (UV) LED grown on a low-dislocation GaN substrate has recently exhibited an internal quantum efficiency as high as 80% [17]. There might be little room for improvement over these values, especially for nonpolar and semipolar oriented devices grown on high quality freestanding GaN substrates.
On the other hand, there is plenty of room for improving the light extraction efficiency. For a bare chip nitride based LED, because of the rather huge difference between the refractive indices of GaN (n=2.5) and air (n=1), the angle of the light escape cone is only 23 degrees, which leads to a meager light extraction efficiency that is as low as 4.18% [18]. The light outside the escape cone is reflected repeatedly inside the device and eventually absorbed by the active region or the electrodes.
Surface roughening procedures could be used to significantly reduce internal loss of light and encourage light escape from the device. FIG. 1 is a schematic cross-sectional illustration of a surface roughened LED, comprising an n-type electrode 10, n-type III-nitride layer 11, III-nitride active region 12, p-type III-nitride layer 13 and p-type electrode 14 that is bonded to a silicon sub-mount 16 via a gold tin bonding 15. A photo-enhanced chemical (PEC) etching is used to roughen the backside 17 of the n-type layer 11, which is a nitrogen-face (N-face) GaN surface. The arrow 18 indicates a possible trajectory for light emitted by the LED. A 130% increase in output power was measured for a surface roughened LED compared with a smooth surface and otherwise identical device [19].
Although surface roughening by PEC etching is a sine qua non for improving light extraction from a nitride based LED, the effectiveness of this technique by and large hinges on the crystal orientation and polarity of the to-be-roughened surface, particularly, the nitrogen face of a c-polar [0001] GaN [21]. As a result, PEC etching could not be applied to surfaces of other GaN crystal orientations and polarity, including a-face (11-20), nonpolar m-face (1-100), and most of the semipolar surfaces. The lack of means for surface roughening has become a major hurdle for nonpolar and semipolar LEDs to achieve higher extraction efficiency and hence higher overall efficiency, and therefore improved roughening techniques are needed to address this issue.